Screen for paper stock and the like



June 13, 1944. o. IMSET SCREEN FOR PAPER STOCK AND THE LIKE Filed NOV. 4, 1939 Patented June 13, 1944 omreo PATENT cr me scREEN'Fom-PAPER STOCK AND THE LIKE otto n'iisrj 'Skdiiri," i1ear oslanorwari vested imlrmuerrrroperr Custodian Application November 4, 1939,'SerialNo. 302,919 In N orw'ay November 5, 1938 The present invention relateslto screens'f'or paper stock, wood p'ul-pand the like- 'Such'screens are usually provided with'me'an's for automatic cleaning of the screen apertures. Themethod most frequently used consists substantially .in generating. .pu'lsa'tory increases" of pressure in the suspension on the discharge side of the screen, whi'ch increases of pressure have the client that the suspension with short periodical tervals is driven back. through the'sc reenaper tures. The pulsatory increases of pressure are usually generated by means of vibrating mem bers disposed on the discharge side of thescreen, or by T oscillating the "screen plates 1 themselves.

The-main object of the inventioh -is' to provide an improved method of operating screens or the kind described; by. means of which method-"a better cleaning 'of the screenholes-unity be"ob-' tained.

The invention comprises further means for carrying out the method according to the in'' vention'.

In the following the invention Wi li be described as applied to 'a screen of theplane typeyit will. however, be understood that the inventionwith advantage maybe used forscreensof other types too, for instance for rotatable screens.

Inscreens of the plane type, thepulsatoryiricreases'of pressure areusually generated bvn'iearis of oscillating diaphragms disposed below the screen plates and enclosing a chamber between the screen plate and the diaphragm. The dia= phragmsare usually, operated from excentricsor cams. If a compulsory-driving of the diaphragm's by means of excentrics is "used, thepressu-re in the said chambers will vary substantially according to a sinus function. A completeworking period consists of a-screenin'g period and-a cle'a'n ing period. The screeningperiodoccurs when the diaphragm is moveddownwardly; the stock being sucked fromthe upper side of the-screen plates through the slitlike apertures and down into the diaphragm chamber. During the'cleaning period the-diaphragm is moved u'pwardly-and forces a minor part of thestockfrom the diaphragm chamber and back through'the screen apertures.

The cleaning effect dependssubstantially on the relative diirerence between the maxi-mum un derpressureduring the screening; period and the maximum overpressure during the cleaning period. During free running ofthe screen, that is when the static altitude of fall; is like "zero and the liquidlevelin thedischarge ho-X isa-at the same height as the-liquid-levelabove the screen;

the pressuresduring the'screening and cleaning period will be'of equal heights, but in the-first case positive and in the second case negative. With increasing capacity of the screen the static altitude of fallmust be increased, i. e. the liquid level in the'dis'charge box must be lowered, with the result that the maximum pressure during the cleaning period will decrease and the'maximum pressure during the screening period correspond ingly increases. Thus the efie'ctivit'y of the cleaning period'will decrease as the static altitude of fall increases. 'According'to the methods hitherto known for operating screens of thiskind every Working period is identical with the subsequent working period.

According to the present invention the mean pressure in the chambers between the screen platesand thediaphragms is automatically raised toa pressure beyond the mean pressure which prevails in the said space when the outlet from the-same-is closed, the period during which'the mean pressure in the said space is raisedibein'g short in relation-to'the interval between two such periods.

The invention will now be described with-reference to the accompanying drawing in which:

Fig. 1 shows a diagram illustrating the pressure variations in the stock below'the screen plates in a -screen operated'accordin-g to the invention,

Fig. z 'diagrammatically shows a plan view-of a' s'cr'een'plant provided with means far carry ing outthe method according to the invention;

Fig. 3 is a'section along the line III-III on Fig; 2, on 'a somewhat enlarged seale, and

Fig. 4 is a sectional view taken along line' l l in Fig. 2.

On Fig. 1 0-'fl'designa'tes the 'zero-lineduring operation and 0'-"fi' the zero-line during free running. The 'wave line indicates the pressure in the suspension on the discharge side of the screen plates. The courseof thiscurvebetween theline's 'ABco'rre's'ponds *to the usualopera-a tion of a plane screen. The hatched parts-Rs and" represent the cleaning-"and screening period' respectively.

The new'feature' according to' the presentin ven'tion' consists therein-that the frequenvpress'ure variations are superposed by a periodical pressure increase, being on Fig. 1 represented by the-section B -C. The period of timecorre sponding'to this section -is in the following description designated by the large cleaning periOd and' the period A' -B-by ,the large screening. period. sent respectivelythecleaning and the screening The hatched parts Br and Srreprethe'deposition of resin increases.

given a comparatively short duration in proportion to the large working period, the ratio being preferably made to lie between A and M In order thoroughly to understand the arrangement of this diagram it is necessary to have a knowledge of certain properties of" the .jets of water or steam from the upper side of the screen plates, which method of cleaning is employed at certain intervals. The diiference is, however, that by the new system the dirt in the slots is driven back to the upper surface of the plates, whereby pollution of the pulp is avoided. Moreover, the large cleaning period according to the new diagram sets in more frequently, so that depositions of resin should not make themselves felt so much as was formerly the case. On washing the plates from their upper surface all dirt removed from the slots is driven down into the diaphragm chamber and thus pollutes the pulp.

pulp-suspension that is to be graded. Taking for example a mass of cellulose with a concentration of, say, per cent, it can be observed that the separate particles of fibre combine together in tangled clumps, provided the pulp is allowed to flow quietly enough. In consequence. of this combination the fibres will therefore be unevenly distributed in the water. When these clumps of .fibres come to be sucked through a, narrow slot there will first take place a partial expression of. water from the clump and when the underpressure beneath the screen plates becomes sufficiently great the whole clump will be sucked through the slot.. Meanwhile it may sometimes happen that such ..a clump comes to lie over the slot at the close of the suction period and that the-suction. pressure is. not strong enough to draw it through the opening. The result will be that at the place in question there will he a layer of fibres largely .deprived of water, whichlayer will choke up the openings and prevent further passage of the pulp. The short cleaning period is fully sufiicient to raise the layer of fibres from the plates and free the apertures from stoppages of this kind, so that the pulp can once more flow through. Meanwhile there are other kinds of impurities which may cause the slot apertures to become closed again. Among these may be mentioned solid particles of such shape and dimensions that they are able to wedge themselves firmly into the apertures. Deposits of resin are also common, and clumps of fibres which originally lay quite loose in the apertures will in course of time become packed in accordingas Against impurities of this kind there is needed quite a different and more vigorous process of cleaning than what takes place in the so-called large screening period. From the above it will be understood that the arrangement shown in the new diagram of operation affords means of attaining a better cleaning of the screen plates and of thereby maintaining the capacity.

' The working diagram shown applies to a single screen plate. Usually a level screen comprises several, for example, twelve such plates and the time of beginning the large cleaning period' may advantageously be arranged in such-a manner that only one of the 12 plates is cleaned atla time. In this way of the screen will in reality be rendered unproductive, but this reduction in effectivity of the screen will be abundantly compensated for by a considerably increased capacity of the plates that are functioning. It is a well known fact the newly cleaned screen plates have a considerably higher capacity thanplates which have been operating for, say, acouple of hours. should in reality correspond to the washing with The large cleaning period As thewashing often takes place while the screen fis in operation it may also happen that particles of dirt floating in the pulp are forced through the apertures, .whereby the pulp is still further polluted.

In order to attain the automatic periodical increase in pressure there may advantageously be arranged in connection with the chambers on the discharge side of the plates pipes for periodical conveyance of liquid under pressure to the said chambers.

In Figs. 1 and 2 isschematically shown a suitable formof execution for appliances for the utilisation of the invention. In this form of execution the aforesaid pipes are connected with ejectors discharging into channels running transversely under the diaphragm chambers and leading from each chamberto a discharge box or channel common for all the chambers.

In Figs. 2 and 3 the numeral I indicates a screen trough in which are placed six screen plates 2. Under the plates are placed ordinary diaphragms 3, which by means of a mechanism, not shown, are keptin oscillating movement in' order to bring about a pulsatory pressure in the diaphragm chambers 4. From these chambers the pulp passes through a channel 5 running transversely under each chamber, which channels discharge into a common discharge box 6.

The apparatus according to the invention comprisesa water supply pipe 1, working in connection with each diaphragm chamber 4, each pipe in the form shown on the drawing being constructed as an ejector, and discharging into a channel 5. By letting the ejectors discharge into the transversely placed channels 5 it isensured that the conversion of the kinetic energy of the liquidto pressure energy will take place with the greatest efiectivity. I

The liquid is conveyed under pressure through pipe 8a into a common supply channel or conduit 8, and then to the injectors. The opening and closing of the separate electors is effected by means of a valve introduced before each ejector, which valves are controlled automatically from a control device acting in common for all of them} In the form of execution shown on the drawing these valves are constructed in the form of a diaphragm 9, for example of rubber, outside of which there is a chamber In each of which is connected by one of the pipes H, Ila, llb, Hc, Nd and Heto a control device [2 common for all the ejectors. The valves operate in the manner that, when liquid is not to be conveyed to them, i. e. in the large screening period, fluid pressure is maintained against the inlet ends of the injectors by keeping the chamber In under pressure.

, When the injectors are to be brought into action,

l0, leaving a slit between the rear end of the nozzle and the diaphragm, through which slit the liquid under pressure flows from the channel 8 into and through the injector and into the channel 5.

The control apparatus l2 may be operated in any suitable and known manner, for example, by use of a rotating member l4 having spaced ports Ma. therein (Fig. 4). By successive movements of member It, the ports Ma successively register with pipes H, Ila, H1), H0, lld and He. In the present instance, there are six pipes but only five ports Ma; consequently, communication between one of the chambers II] and the control device l2 will always be cut on". Therefore, fluid will be allowed to pass from supply chamber 8 through the corresponding ejector I into a channel 5 to produce a large cleaning period in the associated screen plate 2.

As pressure liquid for the ejectors may be used water or, for instance, the pulp in suspension.

Any more detailed description of working of the apparatus will presumably be unnecessary, as it will be at once understood by experts on reference to the above account of the mode of procedure in accordance with the invention.

I claim:

1. A screen for paper stock and the like, comprising a trough, a number of screen plates disposed side by side in the said trough, a number of vertical partitions between and below the said screen plates dividing the space below the screen plates into a corresponding number of separate chambers, a number of diaphragms disposed one below each of the said screen plates, the said diaphragms constituting the bottoms in the said chambers, a vibrating mechanism for giving the said diaphragms and oscillating movement, a discharge box extending along the said trough, a

number of passageways disposed transversely to the trough below the diaphragms and each connecting the chamber between one of the screen plates and the corresponding diaphragm to the said discharge box, a number of injectors in the said discharge box, each extending against the open outlet of one of the said passageways and adapted to deliver a jet of liquid under pressure into the said passageway, thereby forcing the contents of this passageway back into the said chamber and sucking suspension from the discharge box into the said passageway, a housing having a second passageway therein, said second passageway connecting all of said injectors, means for supplying liquid under pressure to said second passageway, a second diaphragm at the outer end of each injector, means cooperating with the outer face of each of said second diaphragms to form a second chamber, the second diaphragm being adapted to close the outer end of the said injector when a pressure fluid is applied to said second chamber and to open the same when the pressure in the said second chamber is decreased, a source of fluid under pressure, a separate pipe connecting each of said second chambers with said last-named source, and a controlling device cooperating with said pipes for relieving fluid pressure from said last-named source upon said second chambers one at a time, to thereby relieve pressure upon the associated second injector diaphragm and permit the flow of liquid from said second passageway through the injector.

2. A screening mechanism for fluid paper stock comprising in combination, a trough for receiving said stock, a plurality of screens disposed side by side in the bottom of said trough, a discharge trough, a conduit for each of said screens, the upper end of said conduit being covered by said screen and the lower end leading into said discharge trough, means for producing pulsating positive and negative pressures in said conduits to cause said fluid stock to flow back and forth through said screens, a nozzle individual to each conduit and disposed in said discharge box for delivering a jet of liquid under pressure into the lower discharge ends of said conduits thereby causing a reverse flow of the contents in said conduits and sucking suspension from the discharge box into said conduits, a source of liquid under pressure, a supply conduit for conducting said liquid to the inlet end of all of said nozzles, a fluid operated diaphragm valve associated with each of said nozzles for controlling the flow of liquid from said supply conduit to the nozzles, a source of fluid under pressure for operating said valves, means for individually connecting said valves with said last-named source, and a controlling device I'or successively admitting said fluid to a number of said valves and for simultaneously preventing the flow of said fluid to the remainder of said valves.

OTTO IMSET. 

